In a field of producing steel plates, it has recently been attempted to continuously carry out casting and rolling for the purpose of energy-saving, increase of yield, labor-saving, stock-saving and improvement of quality.
In a general method of producing cast sheets using the conventional continuous casting process, cast slabs of about 150-300 mm in thickness are produced from molten steel by means of a continuous casting machine and then subjected to hot-rolling and cold-rolling to produce thin steel sheet of about 0.5-2 mm in thickness. This method is excellent in the production yield, labor-saving and energy-saving as compared with a method of obtaining a cast slab from an ingot through blooming. However, when the casting rate is increased to not less than 2.0 m/min in the usual continuous casting machine, not only the smooth casting becomes difficult but also the surface and inner defects of the cast sheet increase, so that it is very difficult to connect the continuous casting machine to the rolling mills at a continuously operating state. Therefore, even when using the continuous casting process, in order to obtain a thin steel sheet, it is required that the slab is subjected to a rough rolling and a finish rolling after it is reheated at a uniform temperature.
If the cast sheets of not more than 30 mm in thickness can directly be produced from molten steel according to the continuous casting, it is possible to omit some procedures from the rough rolling step for obtaining thin steel sheets. Moreover, if thin steel sheets of several mm in thickness can directly be casted from molten steel, the rolling step can considerably be simplified, so much more it is possible to reduce an investment cost and a processing cost.
Under the above situations, there have been made various attempts for directly producing cast steel sheets for thin steel plates from molten steel. For instance, there are a technique described in Japanese Patent laid-open No. 54-61,036, a technique described in Japanese Patent laid-open No. 54-139,835 and the like, but they have not yet been attained to industrial scale. In these techniques, it is particularly difficult to make a broad cast sheet.
FIG. 1 is a schematic view illustrating an embodiment of the apparatus in which such attempts are more improved. This apparatus comprises a pair of endless metal belts 1, 1' arranged opposite to each other and supported by guide rolls 2, 2', 3, 3' and 4, 4' so as to allow their circulation while keeping a gap for holding casting molten metal over a constant distance, a pair of side plates (not shown) for narrowside planes of a casting space pinched between the metal belts 1, 1' and located near the both side edges of the metal belts, metal pads 5, 5' arranged behind the opposed portions of the metal belts, and cooling fluid paths (not shown) provided inside of the metal pads for cooling and supporting molten steel through the metal belts by filmy cooling fluid flows formed by flowing a cooling fluid between the metal belts 1, 1' and the metal pads 5, 5', from nozzles of the paths opening at the pad surfaces side the belts, whereby molten metal 7 is poured into a casting space defined by the metal belts 1, 1' and the side plates from a pouring nozzle 6 and cooled and solidified along the surfaces of the metal belts and the side plates to obtain a cast sheet 8.
However, in the construction as shown in FIG. 1, it is demanded that the size in thickness direction of the molten steel flowpath in the pouring nozzle 6 for supplying the molten steel into the casting space must be small such as several mm to several tens mm and also the thickness of the refractory at the top of the pouring nozzle 6 must be thinned, so that there are such fatal drawbacks that molten steel is solidified in the pouring nozzle 6 to cause the clogging, and the refractory is eroded so as not to endure a long-term continuous service.
As the improved technique for solving the above drawbacks, there have been proposed a combination of casting wheels and belts as disclosed in Japanese Patent laid-open No. 57-32,852 and an apparatus as shown in FIG. 2. In the illustrated continuous casting apparatus, metal belts 1, 1', side plates 9, 9' and rolls 10, 10' and 11, 11' are arranged so that as the casting space defined by a pair of the opposed metal belts 1, 1' and a pair of the opposed side plates 9, 9' advances downward in the moving direction of the metal belts, the thickness of the resulting cast sheet is reduced from a thickness larger than a given thickness up to the given thickness to thereby define a downwardly tapered molten steel holding portion 12a and a subsequent molten steel solidifying portion 12b having a constant thickness corresponding to the given thickness of the cast sheet.
Therefore, according to this continuous casting apparatus, as shown in FIG. 3, the molten steel 14 poured into the molten steel holding portion 12a through a pouring nozzle 13 forms a solidification shell 15 from its surfaces mainly contacting with the metal belts 1, 1', which is led into the molten steel solidifying portion 12b while the thickness t is gradually convegred during the downward movement with rowing and regulated to the desired thickness through the rolls 11, 11'. Then, in this molten steel solidifying portion, as shown in FIG. 4, the solidification shell 15 grows to complete the solidification at the outlet of the lower end of the solidifying portion, which is then drawn out in the form of cast sheet 8.
As mentioned above, the continuous casting apparatus as shown in FIG. 2 is constructed so as to gradually reduce the thickness of the poured molten steel in the downwardly tapered or funnel-like molten steel holding portion 12a, so that it refers to as a belt converging type continuous casting apparatus. In this case, the size in thickness direction at the upper end part of the molten steel holding portion can be made large, so that the problem caused by the use of the thin pouring nozzle 7 as shown in FIG. 1 and also the lower end part of the pouring nozzle 13 can be immersed in the molten steel 14 to pour the molten steel at a non-oxidation state.
However, as mentioned above, in the belt converging type continuous casting apparatus as shown in FIG. 2, it is required to converge the unsolidified cast sheet formed by enveloping the unsolidified molten steel 14 with the solidification shell 15 in the thickness direction at the molten steel holding portion 12a. For this purpose, the converging rolls 11, 11' are arranged in a region changing from the tapered molten steel holding portion 12a to the molten steel solidifying portion 12b of constant thickness so as to apply a converging forces to the unsolidified cast sheet by the rolls 11, 11' through the metal belts 1, 1'. Accordingly, there are caused not only a problem that the unsolidified cast sheet formed by enveloping the unsolidified molten steel 14 with the solidification shell 15 is bulged through the converging force forcedly applied by the converging rolls to cause the breaking but also a problem that deep wrinkle-like defect and cracking are produced in the side surface of the resulting cast sheet.
As seen from the above apparatus, the guide rolls 2, 2', 3, 3' and 4, 4' for supporting the metal belts 1, 1' are usually used to have a diameter of 200-800 mm, while in the metal belts 1, 1' are used steel materials for general structure having a thickness of 0.4-3.0 mm.
However, since such metal belts 1, 1' are used under such a very severe condition that one surface of the metal belt comes into contact with the molten steel, while the other surface comes into contact with filmy cooling water flow from the water cooling pads 5, 5', there are the following various problems. That is, the metal belts 1, 1' are deformed into a wavy form in the widthwise direction. Therefore, the contacting state between the metal belts 1, 1' and the side plates becomes poor and consequently the molten steel penetrates into the generated gap to form fins and the surface of the cast sheet is led to indicate uneven wave form. Moreover, the sliding contact portions between the surfaces of the metal belts 1, 1' and the edges of the side plates are easily injured to promote the aforementioned deformation and fin formation and the considerably shorten the lifetime of the belt, which has caused an obstruction in a direct connection to a rolling equipment.
Furthermore, the metal belt has been endlessly joined by a butt TIG welding. In this case, however, the thermal deformation of the weld zone is large, so that the poor shape of the belt as mentioned above is caused and also cracking is apt to occur in the weld zone, particularly the thermally affected zone, occasionally resulting in the breaking of the belt.
Additionally, the above metal belt has the following problem. That is, as understood from the above, the side plates 9, 9' for the narrow-side planes of the illustrated continuous casting apparatus must be constructed so that by heating the side plates during the casting, particularly at the beginning of the casting, the formation of the solidification shell 15 along the side plate is made later than that of the other (broad-side) solidification shell formed along the side of the metal belts 1, 1' and therefore the rate of growing the thickness of the solidification shell becomes slower. This reason is based on the fact that since a considerable converging is required for casting a cast sheet of, for example, about 30 mm in thickness, if the formation of the solidification shell along the side plates 9, 9' is equal to or faster than that along the broad-side metal belts 1, 1', the cast sheet suffers a compression at the lower part of the converted casting space to generate cast wrinkles thereon and a drawing resistance becomes large and in extreme case the drawing can not be performed.
From the above, the inventors have already proposed apparatuses as disclosed in Japanese Patent laid-open No. 58-32,551 and Japanese Patent laid-open No. 58-32,552 prior to the filing of the invention, wherein it is attempted to slowly form the solidification shell at that portion of molten steel which contacts with the side plate for narrow-side plane by heating the side plates 9, 9' through heaters embedded therein while the inner surface of the side plate is made of refractory, or by radiating a flame of a gas burner to a gap defined between the side plate 9, 9' and a partition plate vertically arranged inside the side plate and spaced therefrom prior to the beginning of the casting.
However, the above prior arts have such a drawback that the surfaces of the metal belts 1, 1' are oxidized to reduce their lifetimes due to the heating of the side plates 9, 9'. This tendency is more remarkable since thinner metal belts are used as the metal belts 1, 1' in view of the cooling effect.
Moreover, such a problem has been found that in case of the above casters, the lubrication between the cast sheet (solidification shell) and the inner surface of the belt is insufficient to cause a seizing as compared with the ordinary continuous casting in which a mold is lubricated with powder by applying an oscillation.
It is, therefore, an object of the invention to overcome the aforementioned various drawbacks I5 involved in the known belt converting type continuous casting apparatus for the production of cast sheets: that is to say,
Firstly, the shape of the side plates is optimizedly designed so as to prevent the occurrence of defects generated on the side surface of the cast sheet and make the drawing resistance as small as possible to hardly produce the narrow-side solidification shell in the molten steel holding portion;
Secondly, the metal belt is designed to have a long-term life without causing a reject quality of the cast sheet and an accident of casting due to deformation of the metal belt;
Thirdly, the metal belt is designed to be suitable for producing cast sheets having excellent antioxidant properties and lubricating properties and hence an improved surface form.